1. Field of the Invention
The invention relates to a process for producing rotationally symmetrical components from a pipe, especially hollow monoblock shafts, the pipe initially having a constant outside diameter and a constant wall thickness, and the rotationally symmetrical component having at least over one area of its entire length an outside diameter which deviates from the constant outside diameter, especially as a smaller outside diameter, and/or a wall thickness which deviates from the constant wall thickness, especially as a greater wall thickness. In addition, the invention relates to a rotationally symmetrical component.
2. Description of Related Art
Rotationally symmetrical components which have different outside diameters and different wall thicknesses over their entire length are used especially in motor vehicles as drive shafts, camshafts, intermediate shafts or gear shafts. Under the aspect of “weight reduction” which is generally becoming more and more important, shafts produced from pipes, so-called hollow shafts, have been used for some time instead of shafts produced from solid bars. There are basically two different types of pipes each of which differ in their production process. Pipes, especially steel pipes, are produced either in a seamless version, i.e., from a solid material without a lengthwise seam, or in a welded version, i.e. from bent sheet metal or steel strip with a lengthwise seam. For rotating components generally welded pipes are used since seamless pipes require concentricity that is not always reliably ensured. In addition, production of seamless pipes is generally more expensive than production of welded pipes.
In order to produce the aforementioned rotationally symmetrical components with different outside diameters and wall thicknesses, at least theoretically there is the possibility of joining several pipes, each having a constant outside diameter and constant wall thickness, into a composite pipe with the desired outside diameter and wall thickness variation. These pipes composed of several individual pipes however generally do not meet the high mechanical requirements imposed on shafts in operation.
Therefore in the prior art, especially in motor vehicles, only monoblock shafts are used, i.e., those shafts which are made from a single piece, in this case from a single pipe. The shaft is generally produced from the pipe using the so-called rotary swaging process at room temperature. Generally, it is desirable that the shaft in its middle area have a wall thickness as small as possible and in one or both end areas a smaller outside diameter and a much greater wall thickness.
In this shaft construction the wall thickness which can be achieved in the end area by the rotary swaging process cannot be arbitrarily increased, but depends on the outside diameter and the wall thickness of the original pipe, and on the outside diameter of the end area of the shaft (material preservation or constant volume). If the end area is to have an especially large wall thickness, it is necessary for the initial material, i.e. the original pipe, to have a large enough wall thickness or a correspondingly large outside diameter. This can then lead to the wall thickness and/or the outside diameter of the original pipe having to be larger such that the wall thickness or outside diameter of the finished shaft in the middle area is larger than desired. As a result, the pipe not only needs the end areas to be worked by rotary swaging, but, in addition, the middle area must be reduced by sinking both in its outside diameter and also in its wall thickness.
Another problem often arises due to the fact that welded pipes cannot be produced with just any wall thickness or with just any ratio of wall thickness to outside diameter. Here the maximum ratio of wall thickness to outside diameter is roughly 1/7. If the pipe is to have an even greater wall thickness or a smaller outside diameter with the same wall thickness, this can no longer be achieved by simple bending of sheet metal or steel strip and subsequent welding of the pipe. In this case, first a pipe with a larger outside diameter and a smaller wall thickness must be produced, i.e. bent and welded, and must then undergo one or more drawing processes, by which the outside diameter and the wall thickness of the pipe is reduced. If several drawing processes are necessary to achieve the desired pipe, generally a heat treatment of the pipe is necessary between the individual drawing processes. By the additional working steps in the production of a pipe, for so-called “drawn” pipes, the price is much higher than for simply welded pipes. The additional cost for “drawn” pipes being up to 30%.
In the prior art, the production of the initially described rotationally symmetrical component from a pipe requires the following steps shown in FIGS. 1A-1D:                Producing a welded pipe with an outside diameter D1 and a wall thickness d1,        Producing a pipe with an outside diameter D2<D1 and a wall thickness d2<d1 by one or more drawing processes,        Sinking an area, preferably the middle area, of the pipe so that in this area the pipe has an outside diameter DM<D2 and a wall thickness dM<d1, and        Working of at least one area, preferably an end area, of the pipe by rotary swaging at room temperature so that in this area the pipe has an outside diameter DE<D1 and a wall thickness dE>d2.        